With continuous development of mobile communications technologies and gradual network upgrade of operators, a global system for mobile communication (Global System of Mobile Communication; hereinafter referred to as GSM) network, a universal mobile telecommunication system (Universal Mobile Telecommunication System; hereinafter referred to as UMTS) network, and a long term evolution (Long Term Evolution; hereinafter referred to as LTE) network may coexist in a practical network. To ensure continuity and quality of a communication service of a user, it needs to ensure that the user can successfully perform handover between different networks.
Currently, handover from the UMTS to the GSM has been standardized, a current mechanism has relatively well solved a problem of algorithm negotiation and key negotiation in a handover process, and a user terminal supporting both UMTS access mode and GSM access mode has appeared in the market and is relatively widely applied. In the 3rd generation partnership project (3rd Generation Partnership Project; hereinafter referred to as 3GPP) standard, a mechanism of handover between the LTE network and the UMTS network is also established. However, handover delay of these existing handover technologies is relatively high.
In a handover solution of handing over from the LTE to the UMTS, a private interface X2-u is introduced to reduce handover delay, where the X2-u interface is an interface between an evolved NodeB (evolved NodeB; hereinafter referred to as eNB) and a radio network controller (Radio Network Controller; hereinafter referred to as RNC). However, this handover solution cannot solve the problem of inter-system key negotiation and algorithm negotiation, thereby resulting in relatively low handover security.